For the sake of cost reduction and miniaturization, on-chip integration is proceeding in the field of short-range wireless communication such as Bluetooth. The demand for on-chip integration is high especially for image rejection filters used for filtering received signals. In on-chip integration, even if the overall area of circuit is reduced, performance needs to be within satisfactory levels.
Conventional image rejection filters that include inductors consumed large areas. The area of circuit can be reduced by sharing the image rejection filter with the matching circuit of power amplifier. However, power amplifiers with high output have low impedances. Therefore, it is difficult to make the impedance of matching circuit match with the high impedance of image rejection filters. If matching circuits with the impedance of image rejection filters are used, it would put a limit to the output of power amplifiers that can be used. Also, it is difficult to improve the overall performance, if some sections of circuits are shared.
Generally, the distance between the resonant frequency and the anti-resonant frequencies is inversely proportional to the quality factor of the LC resonant circuit. Thus, if on-chip inductors with low quality factors are used, it is difficult to place the resonant frequency and the anti-resonant frequency in close frequency ranges. If the image frequency is within the vicinity of the reception frequency, it becomes difficult to eliminate interfering waves in the image frequency.